SnO2–Fe3O4 nanocomposites for the photodegradation of the Congo red dye

Synthesis of SnO2–Fe3O4 nanocomposites was conducted. The purpose of this study was to obtain the SnO2–Fe3O4 nanocomposites, the effectiveness of photodegradation Congo red by SnO2–Fe3O4 nanocomposites and determine the kinetics of photodegradation. The XRD analysis showed that the SnO2–Fe3O4 nanocomposite best mass ratio was 1:1 based on the highest intensity of characteristic angle (2θ), which is 26.54° and had the smallest crystal size, which is 9.662 nm. Based on the UV-Vis DRS result, the SnO2–Fe3O4 nanocomposites bandgap value was 2.3 eV. The magnetization saturation value of SnO2–Fe3O4 nanocomposites was 64.96 emu/g. The morphology of SnO2–Fe3O4 nanocomposites showed by the TEM image, where the dark spots spread in the lighter areas. The surface of SnO2–Fe3O4 nanocomposites characterized by SEM with the result was bumpy surface and many pores. The EDS result of SnO2–Fe3O4 nanocomposites confirmed the presence of Fe, Sn, and O elements. The functional group of SnO2–Fe3O4 nanocomposites showed by FTIR data, the stretch band of Sn–O characteristics showed at wavenumber 590 cm−1, and the stretch band of Fe–O showed at wavenumber 563 cm−1. The optimum condition of nanocomposites at a contact time of 90 min and the optimum concentration of 18 mg/L showed that the percent of photodegradation was 50.76%. The photodegradation rate of SnO2–Fe3O4 was fitted to Pseudo-second-order.


Introduction
SnO 2 was reported by Wu et al. [1] to possess a photocatalytic ability because it degrades dyes and other organic compounds. This photocatalytic ability is usually associated with semiconductors and it is improved through modification and the development of composites. Bharati et al. [2] stated that Fe 3 O 4, a nanomagnetic material is a semiconductor that provides the best effect in photodegradation and stability in water media. Gandhoor et al. [3] showed that Fe 3 O 4 was classified as a semiconductor with a bandgap of 0.3 eV. Therefore, it was described to possess the strongest magnetic properties among other iron oxides.
Several methods are utilized in the synthesis of semiconductor oxide nanoparticles, namely sol-gel [4], flame spray [5], and hydrothermal [6]. The hydrothermal method is a process where water and heat are utilized to convert solutions into crystals. It is usually carried out in a closed system to prevent the loss of solvent when heated above its boiling point [7]. One of the advantages of the hydrothermal method is that it produces higher crystallinity and purity [8]. According to Wang et al. [9], which synthesized SnO 2 -Fe 3 O 4 using the hydrothermal method, it also showed its importance. In this study, two methods of SnO 2 -Fe 3 O 4 synthesis were combined, namely hydrothermal and coprecipitation. By combining the two processes, it was expected that the resulting nanocomposite possesses high purity and magnetic properties.
Recently, the industrialization process, technological advances, and the declining availability of clean water have become major concerns. Various contaminants such as toxic chemicals, phosphates, heavy metals, dyes are present, causing a decrease in water quality [10]. Among the pollutants that have been reported, dyes are the primary pollutant in wastewater and are often present in large quantities from various sources [11]. Specifically, cationic dye such as Congo red is hazardous because it is toxic, carcinogenic, and causes various problems. This dye is also difficult to naturally degrade in the environment [12]. Therefore, it is highly recommended to remove pollutants from wastewater before being discharged into water resources or the environment as well.
Several researchers have experimented with different dye degradation processes, including the use of microorganisms [13] and photocatalysts [14]. The photocatalyst method is a fairly efficient process compared to others, because it decomposes dye compounds into harmless chemicals such as H 2 O and CO 2 .

Materials
The materials used in this study were distilled water, hydrochloric acid (HCl) (37-38% purity), sulfuric acid (H 2 Table 1. The solution was adjusted to pH 2 and was placed in Teflon then it was heated hydrothermally in an oven at 150 C for 12 h. The precipitate obtained was dried at 70 C for 24 h [9]. The prepared nanocomposites were characterized using spectrophotometer FT-IR Thermo Scientific Nicolet iS10, Scanning Electron Microscope-Energy Dispersive X-ray Spectrometry (SEM-EDS) VEGA 3 TESCAN, and UV-Vis Orion Aquamate 8000. Also, UV-VIS Diffuse Reflectance Spectroscopy (UV-Vis DRS) UV-1700 Series, Transmission Electron Microscopy (TEM) JEOL JEM 1400, Vibrating Sample Magnetometer (VSM) Lakeshore 74004, X-Ray diffractometer (XRD) Rigaku Miniflex-600 were utilized.

Photodegradation study
Photodegradation batch studies were conducted in a UV light photoreactor with a wavenumber of 254 nm and a power of 14 W (Philips). Determination of the optimum conditions was conducted in sequence by each variable. 0.03 g of nanocomposite was mixed with 30 mL of Congo Red with initial dye concentrations variations of 6, 10, 14, 18 and 22 mg/ L and H 2 O 2 volume variations of 0.1, 0.2, 0.3, 0.4, 0.5, 0.75 mL. Subsequently, it was irradiated in a UV photoreactor with irradiation time variations for 30, 45, 60, 75, 90, and 105 min and stirred using a magnetic stirrer. After that, the solution was separated from the nanomagnets using a permanent magnet. The absorbance of the filtrate was measured at the maximum wavelength of Congo red (449 nm). The photodegradation using SnO 2 and SnO 2 -Fe 3 O 4 nanocomposite without UV lamp irradiation was also carried out as a control experiment with the same procedure.

Characterization using XRD
Synthesis of SnO 2 in this research was conducted according to the standard reaction. Then the resultant was compared with the standardissue by Joint Committee on Powder Diffraction (JCPDS) No. 88-0287. Based on Figure 1 (a) which showed the diffraction pattern of SnO 2 nanomagnetic X-rays, where the peak widened at the centre of 2θ at 26.52, 32.1, and 51.8. Data from the XRD characterization results and particle size calculations for SnO 2 using the Debye-Scherer formula, showed that the resultant size of SnO 2 was 2,829 nm. This indicated that it has a nanoparticle shape because it has a particle size that was below 100 nm.
The magnetic properties of the synthesized nanoparticles were characterized using XRD which showed a diffraction angle of 2 θ, peak intensity, and different types of phases. Furthermore, it was conducted by compa x ring the peaks formed on the Fe 3 [17].
Based on Figure 1 the diffraction pattern of Fe 3 O 4 nanomagnetic Xrays in which the peak widened at the centre were 35.86 and 57.2, indicating the successful synthesis of magnetite. Then, data were obtained from XRD characterization and particle size calculations for magnetite nanoparticles using the Debye-Scherer formula. The resultant size of the magnetite nanoparticle was 13.66 nm, which showed that Fe 3 O 4 was a nanoparticle because it has a particle size that was below 100 nm and the crystal size of 122.66 nm.  The synthesis of SnO 2 -Fe 3 O 4 nanocomposite with various mass ratios (1:1), (1:2) and (2:1) was carried out using the hydrothermal method. Based on Figure 1, the 2θ angle in the SnO 2 -Fe 3 O 4 (1:1), (1:2), and (2:1) composite had almost the same characteristics as that of SnO 2, especially at 26.52 . The peak intensity of the SnO 2 -Fe 3 O 4 composite was observed to decrease with the addition of SnO 2 in the composite. Based on Wang et al. [9], the composite in comparison with the intensity decreased with the SnO 2 peak covered by the crystallite of magnetite nanoparticles. From the XRD results the best ratio of SnO 2 -Fe 3 O 4 synthesis occurred at a mass ratio of 1:2, showing the highest intensity around the angle of 2θ ¼ 26.74 and 34.08 . Based on the data from XRD characterization and particle size calculations for SnO 2 -Fe 3 O 4 using the Debye-Scherer formula the obtained size of SnO 2 -Fe 3 O 4 (1:1) was 9.833 nm. While that of SnO 2 -Fe 3 O 4 (1:2) was 9.662 nm and SnO 2 -Fe 3 O 4 (2:1) was 13.543 nm which showed that SnO 2 -Fe 3 O 4 was a nanoparticle because it had a particle size that was below 100 nm.

Characterization using UV-VIS DRS
SnO 2 , Fe 3 O 4 and SnO 2 -Fe 3 O 4 nanocomposites were analyzed using UV-Vis DRS. The purpose of this was to determine the value of the energy bandgap and to observe the changes that occurred in SnO 2 when it was combined with Fe 3 O 4 . The energy band gap is the distance between the two bands in a semiconductor, i.e., the distance from the valence, to the conduction band. Characterization using UV-Vis DRS was carried out by providing energy at 200-800 nm. Causing excitation of electrons from the valence to the conduction band, then the electrons returned to their ground state by transmitting a certain amount of energy, which was proportional to that of the bandgap. The results of measuring the bandgap energy using the UV-Vis DRS are presented in Figure 2.
Based on Figure 2, the bandgap energy for SnO 2 was 3.05 eV, and according to Huda et al. [15], SnO 2 is a semiconductor with an energy band gap of 3.160 eV. This showed that the resulting energy band gap was smaller than the standard for SnO 2 , which is 3.6 eV. Furthermore, the synthesized Fe 3 O 4 had bandgap energy of 2.7 eV, and that of the SnO 2 -Fe 3 O 4 nanocomposite was 2.3 eV, therefore it was concluded that the SnO 2 -Fe 3 O 4 nanocomposite was in the semiconductor range. The change in the bandgap energy in the nanocomposites was due to the effect of those in the synthesized Fe 3 O 4 which was smaller than that of SnO 2 . The bandgap value of the SnO 2 -Fe 3 O 4 nanocomposite which was smaller than SnO 2 affected the photodegradation power. The smaller the bandgap size, the greater the photocatalytic properties. According to Wei et al. [18], a good photocatalyst is that which has a low bandgap, because the area with a wider wavelength causes UV absorption and results in more electrons and holes during the photocatalytic reaction process.

Characterization using VSM
This characterization was carried out using a VSM tool, and it was aimed to determine the magnitude of the magnetic properties due to changes in the external field of Fe 3 O 4 . And also to determine changes in the magnetic field of SnO 2 -Fe 3 O 4 nanocomposites and the alterations after being applied as a photocatalyst. The information obtained was in the form of magnetic properties as described by the hysteresis curve, which stated the relationship between magnetization (M) and the external magnetic field (H). The hysteresis curve resulting from the VSM characterization is shown in Figure 3.
Based on Figure 3, Fe 3 O 4 had the highest saturation magnetization value at 78.76 emu/g. These results changed with the nanomagnetic saturation moment of pure Fe 3 O 4 , which was 92 emu/g [19]. The saturation magnetization value was influenced by the presence of an impurity on the magnetic particles. There was a decrease in the saturation magnetization value of the SnO 2 -Fe 3 O 4 nanocomposite which was smaller than that of Fe 3 O 4 . The decrease was caused by the addition of nonmagnetic SnO 2 which coated Fe 3 O 4 . According to Kuppan et al. [20] SnO 2 is a particle that has super nonmagnetic properties. The decrease in the saturation magnetization value was also influenced by the density of the particles which resulted in easy interaction that led to a magnetic moment. This is because the denser the particles, the greater the number of the magnetic moment. Based on the data, the VSM nanocomposite had a saturation value of 64.96 emu/g, which means that it had good superparamagnetic properties. Although SnO 2 -Fe 3 O 4 nanocomposites had lower magnetic saturation values than Fe 3 O 4 , they still possessed the magnetic properties needed for separation. SnO 2 -Fe 3 O 4 nanocomposite was applied as a photocatalyst to degrade the Congo red dye. The role of Fe 3 O 4 present in it was to provide magnetic properties in order to aid the separation of SnO 2 from water, for it to be reused in the next photocatalytic process. After the photodegradation of the nanocomposites, the VSM measurement was again  carried out to observe the changes in the magnetic saturation value. Which discovered that the magnetic saturation value of SnO 2 -Fe 3 O 4 nanocomposite after photodegradation was not too different than before it, at 63.40 emu/g. This proves that the SnO 2 -Fe 3 O 4 nanocomposite is environmentally friendly because it has the potential of being reused for subsequent photodegradation.

Characterization of SnO 2 -Fe 3 O 4 nanocomposites using TEM
SnO 2 -Fe 3 O 4 nanocomposites were also characterized using TEM. The purpose of this was to determine the morphological properties of the SnO 2 -Fe 3 O 4 nanocomposite by observing the crystal structure, identifying the defects, determining the particle size, analyzing the interface, and the microstructure. The results of the TEM analysis are shown in Figure 4.
Based on the TEM results above, the morphological or inner structure results showed that the nanocomposites had either aggregation or a unified structure. In Figure 4 the darker centre was the iron oxide nanoparticles and the bright part was the tin oxide, the results are in agreement with that of Nouri and Sargolzaei [21], where the darker dots appeared to spread out over the bright areas. Figure 4 shows the presence of a material that had a crystallographic field indicating SnO 2 . From this figure, it was also observed that there was an aggregate phase at the darker colour intensity at several points which showed the aggregation of Fe 3 O 4 particles as shown in the TEM results of Wang et al. [22]. From the TEM results on Fe 3 O 4 , a large agglomeration and particles that tended to form clusters were discovered. This analysis was also supported by the condition of Fe 3 O 4 nanoparticles which are prone to clumping due to their strong magnetic force. Based on the results of calculations by measuring the diameter at several particle points, the size of the SnO 2 -Fe 3 O 4 nanocomposite averaged 6.5 nm indicating the size of a nanoparticle.

Characterization using SEM-EDS
The surface of the SnO 2 -Fe 3 O 4 nanocomposite was observed using SEM. As shown in Figure 5 (a). SnO 2 , Figure 5  Based on the SEM results in Figure 5 (a), the SnO 2 structure was in the form of chunks, which had a non-smooth surface and formed agglomeration. Chu et al. [23] reported similar things in their research, where SEM images showed the results were in the form of aggregates and were composed of particles with a diameter of 10-20 nm Figure 5 (b) showed the surface of Fe 3 O 4 which consisted of small, uneven particles and the presence of gaps in the pores. The Fe 3 O 4 particles in the SEM results were not uniformly spherical and they formed agglomeration. This is because the particles have strong magnetic properties which tend to draw closer to one another. The morphology of the synthesized SnO 2 -Fe 3 O 4 nanocomposite is represented in Figure 5 (c), showing that the gap between the pores was bigger than those in Figure 5 (b). This is due to the presence of SnO 2 coating on the composite forming a core-shell structure. The results of Wang's study [24] showed that SnO 2 coats and sticks to the outside of Fe 3 O 4 . The EDS results are observed in Table 2.
Based on the EDS results in Table 2 for SnO 2 , the main constituents were Sn and O, with the respective percentages of 72.49% and 21.39%. And the major constituents of Fe 3 O 4 were Fe at 43.57% and O at 36.9%. In the SnO 2 -Fe 3 O 4 nanocomposite, there was an increase in the percentage of Fe by 56.52% indicating that this metal is the major constituent of the nanocomposite, while the percentage of Sn element was 13.07%. This causes the pore gap size in the nanocomposite to become smaller. Based on the results of the EDS, impurities were also discovered in the form of F, C, Co, Na and Si. The presence of these impurities comes from the coating process and also during the nanocomposite washing process.

Characterization using FTIR
The synthesized SnO 2 -Fe 3 O 4 and SnO 2 nanocomposites were also characterized using FTIR. This was aimed to identify the functional groups present in the synthesized SnO 2 and SnO 2 -Fe 3 O 4 nanocomposites. The FTIR spectrum is presented in Figure 6. Figure 6 showed that there was a significant change in the IR spectrum. In the FTIR spectrum of SnO 2 nanocomposites shown in Figure 6 (a) an absorption band at 597 cm À1 was observed which was the peak characteristic of the Sn-O strain vibration on Sn-OH. While the FTIR spectrum of SnO 2 -Fe 3 O 4 nanocomposites shown in Figure 6 (b), produced a wide absorption band that was observed at 3215 cm À1 on the O-H strain vibration. With the presence of an O-H absorption band that had a low transmittance, the crystal formation process was described as almost perfect. The absorbance band at 1626 cm À1 was described to represent the vibrations of the hydroxyl groups on the surface. The broad absorption peaks appearing between 400 cm À1 and 600 cm À1 were defined as the strain between metal and oxygen (M-O). The absorption band at 507 cm À1 was related to the vibration strain of Fe-O on Fe 3 O 4 nanoparticles. Another absorption band at 590 cm À1 was characterized as the Sn-O strain band in SnO 2 [25]. Figure 6 also shows that the changes and shifts were observed in the absorption bands within several spectra on the SnO 2 -Fe 3 O 4 nanocomposites after photodegradation. In each spectrum, there was an OH group that appeared in the 3200-3600 cm À1 region. In Figure 6 (b) a water absorption vibration appeared on the O-H at 3212 cm À1 , however in Figure 6 (c) the O-H vibration increased in the percent transmittance. At 590 cm À1 , there was an increase in the transmittance of the nanocomposite before photodegradation (48.067%) and after (75.937%). At 563 cm À1 , there was a change in the wave number of the nanocomposite after photodegradation to 507 cm À1 which indicated the effect of this process.
The results of characterization using XRD, UV-Vis DRS, VSM, TEM, SEM-EDS and FTIR showed that the SnO 2 -Fe 3 O 4 nanocomposites were successfully synthesized by a combination of hydrothermal and coprecipitation methods. The SnO 2 -Fe 3 O 4 nanocomposite had magnetic properties with a crystal size of 9.833 nm and an average particle size of 6.5 nm therefore, it was said to possess a nano size.

Point zero charge (PZC)
SnO 2 -Fe 3 O 4 and SnO 2 nanocomposites were also analyzed by conducting a pHpzc determination test. The purpose of this was to ascertain the charge properties on the surface of the SnO 2 -Fe 3 O 4 and SnO 2 nanocomposites. pHpzc is a condition at a pH point, in which the surface of the nanocomposite has no charge i.e., its value is zero. This analysis Based on Figure 7, the SnO 2 -Fe 3 O 4 nanocomposite showed that there was no change between the initial pH value and that of the final at 6. SnO 2 did not experience changes in the initial and final pH value at 7. According to Azeez et al. [26] the surface of the material is positively charged when the pH value is <pHpzc to allow the material to degrade an anionic compound. Meanwhile, the surface of the material becomes negatively charged when the pH value is > pHpzc, in order to degrade cationic compounds. In this study, the material used was applied for the photodegradation of the anionic congo red dye.

Variation in contact time and determination of kinetic parameters on the photodegradation of Congo red
The variation of contact time was carried out to determine the optimum time for the photodegradation of the congo red dye by SnO 2 -Fe 3 O 4 nanocomposites. The SnO 2 -Fe 3 O 4 and SnO 2 nanocomposites did not only act as photocatalysts, they also work as dye adsorbents during degradation. This is because both the SnO 2 -Fe 3 O 4 nanocomposites and SnO 2 have a wide surface and a porous structure [27]. Therefore, the photodegradation process of the SnO 2 -Fe 3 O 4 nanocomposite without UV    irradiation was carried out using the adsorption method. The following is the curve showing the effect of time on the amount of congo red dye as shown in Figure 8.
In Figure 8 the effectiveness of reducing the concentration of congo red dye with UV irradiation was observed to be higher than when it was without it. The percentage of reduction of the congo red dye with and without UV irradiation increased with time, this was because the longer the UV radiation time, the more photons or energy that hits the surface of the photocatalyst. The duration of irradiation at the time of photodegradation described the interaction time between the photocatalyst and UV light which produced free electrons (e À ) and holes (h þ ) that later formed a redox reaction for the degradation of a compound. Nanocomposites have a limited ability to degrade a compound after reaching the optimum time. Based on the data obtained in Figure 8, the optimum time for photodegradation of congo red with and without UV irradiation was the same, i.e., at 90 min. At this optimum time, a balance had been reached to effectively reduce the dye concentration at 52.481%, 49.856% and 36.436%, respectively. This is because the ability of the photocatalyst to excite electrons from the valence band to that of the conduction did not increase and the resulting OH was widely used to degrade intermediates [28].
Based on the data obtained in Figure 8, it was observed that the effective reduction of congo red dye by SnO 2 -Fe 3 O 4 nanocomposite with UV light irradiation was greater than when reducing the concentration of SnO 2 using lamp irradiation and SnO 2 -Fe 3 O 4 nanocomposite without UV light irradiation (control). Based on these data, it was stated that the presence of UV irradiation accelerated the decrease in the concentration of congo red dye.
The percentage decrease in the concentration of the congo red dye by the SnO 2 -Fe 3 O 4 nanocomposite without UV irradiation was smaller than when it was with it. Therefore, it was stated that only adsorption occurred during photodegradation without UV irradiation. According to Paramarta [29], SnO 2 is a metal oxide that shows its potential as an adsorbent. From the two statements, it was concluded that the photodegradation of the congo red dye by the SnO 2 -Fe 3 O 4 nanocomposite using UV light irradiation, involved the adsorption process. Based on the time variable data, it was observed that the longer the photodegradation process, the higher the results.
Data on the effect of photodegradation time obtained was utilized to calculate the rate of degradation using the Pseudo-first-order and Pseudosecond-order model [30] as shown in Eqs. (1) and (2) as follow: log ðQ e À Q t Þ ¼ log Q e À k 1 2:303 t (1) Where Q e and Q c were photodegradation capacity at equilibrium (mg/g), respectively, k 1 (min À1 ) and k 2 (g mg À1 min À1 ) were pseudo-first-order and second-order kinetic photodegradation rate constant, respectively, and t was a time (min). The photodegradation rate is the ability of the SnO 2 -Fe 3 O 4 and SnO 2 nanocomposites to reduce congo red dye at a certain concentration and time. The kinetic model calculation data is presented in Table 3.
In Table 3 Pseudo-second-order also showed that the value of equilibrium photodegradation capacity (Q e ) in the experiment is not different from the value of Q e obtained from the equation. Based on the data above, the photodegradation of SnO 2 -Fe 3 O 4 nanocomposite with UV irradiation, SnO 2 with UV irradiation, and SnO 2 -Fe 3 O 4 nanocomposite without UV irradiation followed the Pseudo-second-order equation. The highest value of constant (k) was observed in the photodegradation process of SnO 2 -Fe 3 O 4 nanocomposite with UV light irradiation. Knowing that the greater the value of the constant, the faster the reaction rate. A second-order kinetic model was also obtained by Nouri et al. [21] using SnO 2 -Fe 3 O 4 nanocomposite for phenol photodegradation, while Vinosel et al. [25] utilized SnO 2 -Fe 3 O 4 nanocomposite to degrade organic compounds following a first-order kinetic model. Based on the photodegradation kinetics, SnO 2 -Fe 3 O 4 nanocomposite followed the pseudo-second-order equation.

Variation of Congo Red's initial concentration of dyes
Variations in the initial concentration of congo red dye were carried out to determine the optimum concentration of photodegradation which occurred while utilizing SnO 2 -Fe 3 O 4 nanocomposite. The curve for determining the optimum conditions for photodegradation of congo red dye on the effect of concentration is shown in Figure 9. Figure 9 shows that the optimum concentration of photodegradation of Congo red dye with or without UV irradiation was 18 mg/L. At this condition, an equilibrium of concentration reduction was gradually reached at 50.768%, 44.600% and 36.723%. Based on Figure 9, a comparison was made between the percentage of concentration of Congo red dye that was reduced with and without UV irradiation. For the photodegradation process with UV irradiation, the concentration reduction was higher than without radiation. This is in accordance with Alshabanat's research [31] which stated that the UV lamp produced OH which helped to degrade Congo red dye. While treatment without UV irradiation only adsorbed the dye. Based on the results obtained, the concentration affected the percentage of photodegradation, I.e., the higher the concentration of congo red, the lower the percentage of photodegradation.

Variation of volume addition of H 2 O 2
Variations in the addition of H 2 O 2 volume was aimed at determining the optimum volume of H 2 O 2 that causes an increase in the reduction of the concentration of congo red. Congo red dye photodegradation effectiveness curve on the effect of adding H 2 O 2 volume is observed in Figure 10.
This drastic increase was due to the higher H 2 O 2 concentration that was added to the photodegradation of congo red by the SnO 2 -Fe 3 O 4 nanocomposite. This led to an increased number of electrons in the conduction band which bounded to H 2 O 2, therefore preventing the occurrence of recombinant charges (e À ) and (h þ ). OH continued to increase at the addition of 0.5 mL of H 2 O 2, while at 0.75 mL of H 2 O 2 , the percentage of effectiveness decreased because at that volume the less reactive HO 2 had started to form.
The optimum condition for the addition of H 2 O 2 was 0.5 mL. According to Saha et al. [17], the effectiveness of the photodegradation decreased with the addition of an excess volume of H 2 O 2 due to the formation of less reactive HO 2 which reacted with OH , causing it to decrease. Based on the results obtained, it was observed that adding H 2 O 2 to the photodegradation process while using SnO 2 -Fe 3 O 4 nanocomposite had no effect.

Photodegradation mechanism
Photodegradation usually occurs on the semiconductor photocatalyst's surface after exposure to photons of light (UV). In the beginning, congo red adsorption on the surface of the nanocomposite had occurred through the electrostatic attraction between the sulfoxide (-SO 3 -) of the congo red molecule with the positive charge site of the nanocomposite [32]. Hereafter, photo-induced electron-hole pairs from nanocomposite can be produced during UV irradiation. When the photon energy is equal or higher to the bandgap of the semiconductor photocatalyst, electrons are promoted from the valence band to the conduction band, leaving an electron vacancy or hole in the valence band [33].  [34]. Photogenerated holes can react with OH ions to generate potent oxidizing hydroxyl radicals OH , which can eminently degrade congo red as well [35]. In addition, the holes can also directly degrade when taken by organic compounds on the surface of the SnO 2 -Fe 3 O 4 composite.

Conclusion
Based on the results of the study conducted, the following conclusions were obtained: 1. SnO 2 -Fe 3 O 4 nanocomposite was successfully synthesized by the combined hydrothermal and coprecipitation methods. This was shown by the results of characterizations that were performed using XRD, UV-Vis DRS, VSM, TEM, SEM-EDS and FTIR. 2. Optimum photodegradation of congo red using SnO 2 -Fe 3 O 4 nanocomposite was obtained at 90 min, when the concentration was 18 mg/L, and it produced a 50.76% degradation. Also, the results showed that the addition of H 2 O 2 volume did not affect the photodegradation. 3. Congo red photodegradation kinetics by SnO 2 -Fe 3 O 4 nanocomposite was according to the pseudo-second-order equation.

Author contribution statement
Muhammad Said, Poedji Loekitowati Hariani: Conceived and designed the experiments; Contributed reagents, materials, analysis tools or data; Wrote the paper.
Widya Twiny Rizki: Performed the experiments; Analyzed and interpreted the data; Wrote the paper.
Wan Ryan Asri: Analyzed and interpreted the data; Wrote the paper. Desnelli Desnelli: Performed the experiments; Contributed reagents, materials, analysis tools or data.
Addy Rachmat: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data.

Data availability statement
Data will be made available on request.